This invention relates to a method for molding blocks of water soluble alkali metal halides such as sodium and potassium chloride and to a novel release agent which facilitates the release of the blocks from the mold. For the purposes of this invention, the meaning of the term salt block will include blocks of all such water soluble alkali halides, especially those which have utility in the recharging of ion exchange resins in water softening systems. More particularly, this invention relates to an improved method for molding salt blocks whereby the wear of the block press mold box is minimized. More particularly, the invention relates to a method for molding salt blocks wherein the block release agent is water-soluble and therefore does not generate an unsightly scum in brine tanks used in water softening systems.
The most common method for softening water is the cation exchange method in which hard water is brought into contact with beads of a cation exchange resin whereby sodium ions in the resin are replaced by the calcium and magnesium ions that were in the water and the sodium ions are taken up by the water. When the resin is essentially depleted of sodium ions it is recharged by passing brine through the bed of resin beads to remove the calcium and magnesium ions as the soluble chlorides and replace them again with the sodium ions. To generate the brine, sodium chloride in the form of pellets or blocks is placed in a tank and is dissolved by allowing it to sit in a measured amount of water in the tank for a sufficient time before each recharging cycle.
A salt block is a compressed mass of sodium chloride and additives and is commonly made in two sizes--25 and 50 pounds. The approximate dimensions of the 50 pound block are 8" wide.times.8" long.times.11" high. It resists handling and shipping damages and it resists disintegration when stored in brine.
In the production of salt blocks, a release agent is required to minimize wear and damage to the inner surfaces of the block press mold box. These surfaces are commonly those of a stainless steel liner of the box. Calcium stearate is believed to be the most commonly used release agent in the salt industry. The residue of calcium stearate on the blocks of salt that are dissolved in a brine tank is insoluble, however, and the accumulation of said residue forms an unsightly surface scum in the tank. It is highly desirable, therefore, to find a material that has suitable release properties when on the surface of a salt block in the press mold box but does not form a scum when the salt is dissolved in a brine tank.